Its like buying any other used vehicle. Search for a low mileage unit, with few (if any) mechanical and body problems. Look in areas where they don't salt the roads in the winter (a lot of the US Western states, for example). There are good deals out there and if you are willing to do as much work as possible, you can end up with a long lasting Sprinter.
My 2002 is holding up great and I drive it for most normal trips. I work on mine and use the right fluids and maintance intervals. I prefer the simpler emissions on the T1Ns - I also have a DAD unit to help keep it running. I also help out other T1N owners local to me (I'm not a shop, so I don't charge anything for my time - as a result, I've learned a lot and own a lot of specialty tools for the T1Ns).
If you are not willing to learn how to maintain as much as possible, any older vehicle is a poor choice for you. There is a lot of great info on this site if you know how to find it.